In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation, contributing to the mass-scale production of 0.25 micron rule and subsequent devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. Nos. 4,491,628 and 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have attempted test fabrication on the 0.15 micron rule and started investigation on the 0.13 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF excimer laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. Polymers having benzene rings have a somewhat improved transmittance in proximity to 160 nm wavelength, which is far below the practically acceptable level. It was found that in single layer resists, reducing carbon-to-carbon double bonds as typified by benzene rings and carbon-to-oxygen double bonds as typified by carbonyl groups is essential for insuring a light transmittance (see International Work Shop 157 nm Lithography MIT-LL, Boston, Mass., May 5, 1999). It was reported in J. Vac. Sci. Technol., B17(6), November/December 1999 that introduction of fluorine is effective to improve transmittance. A number of fluorinated polymers for resist compositions were proposed in J. Photopolymer Sci. and Technol., Vol. 13, No. 4 (2000), pp. 657-664 and pp. 451-458. However, the transmittance of these polymers does not reach that of polyhydroxystyrene and derivatives thereof adapted for KrF exposure and poly(meth)acrylic derivatives and polycycloolefin derivatives adapted for ArF exposure.
As is known in the art, the bi-layer resist method is advantageous in forming a high-aspect ratio pattern on a stepped substrate. It is also known that in order that a two-layer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF laser exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are blocked with t-BOC groups, in combination with a photoacid generator (see JP-A 6-118651 and SPIE vol. 1925 (1993), p. 377). Positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexylcarboxylic acid is substituted with an acid labile group were proposed for ArF laser exposure (see JP-A 10-324748, JP-A 11-302382, and SPIE vol. 3333 (1998), p. 62). Positive resist compositions comprising a silsesquioxane having hexafluoroisopropanol as a dissolvable group as a base were proposed for F2 laser exposure (see JP-A 2002-55456). The above polymer bears in its backbone a polysilsesquioxane containing a ladder skeleton produced through polycondensation of a trialkoxysilane or trihalosilane.
Silicon-containing (meth)acrylates were proposed as a resist base polymer having silicon pendants (see JP-A 9-110938, J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), pp. 435-446).
The silicon-containing polymer of the (meth)acrylate type has the drawback that its resistance to dry etching with oxygen plasma is weak as compared with the silsesquioxane polymer. A low silicon content and a different polymer main skeleton account for this weak dry etching resistance. The (meth)acrylate of siloxane pendant type also has the drawback of poor developer wetting that it is likely to repel a developer solution. SPIE vol. 3678 (1999), pp. 214, 241 and 562 describes a polymer comprising (meth)acrylate of the trisilane or tetrasilane pendant type having an increased silicon content and a silicon-containing substituent which can be eliminated with acid so that the polymer is improved in alkali dissolution. Despite absorption at a wavelength of less than 200 nm due to silicon-to-silicon bonds, this polymer is so transparent relative to KrF excimer laser beams of 248 nm that it is a useful polymer having a silicon-containing acid-eliminatable group and improved etching resistance. Besides, silicon-containing acid labile groups other than the foregoing have been investigated as reported in SPIE vol. 3678 (1999), p. 420.
The applicant/assignee proposed novel acid labile groups having silicon introduced therein in JP-A 2001-278918 and JP-A 2001-158808 (U.S. Pat. No. 6,492,089). These polymers have the advantages of an improved acid elimination ability and prevention of T-top profiling as well as the advantageous feature that they are fully transparent at the wavelength of ArF excimer laser because of the inclusion of a carbon atom between silicon atoms and the exclusion of a silicon-to-silicon bond.
Drawbacks of the silicon-containing resist compositions include poor line edge roughness and the generation of scum on substrates. Silicon-containing groups, especially alkyl-substituted silyl groups are extremely hydrophobic so that they interfere with development with alkaline water and incur swelling. As a result, the line edge roughness is exaggerated, and residues following dissolution become scum on the substrate or resist pattern.